1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Base stations in wireless communication systems provide wireless connectivity to users within a geographic area, or cell, associated with the base station. In some cases, the cell may be divided into sectors that subtend a selected opening angle (e.g., three 120° sectors or six 60° sectors) and are served by different antennas. The wireless communication links between the base station and each of the users typically includes one or more downlink (DL) (or forward) channels for transmitting information from the base station to the mobile unit and one or more uplink (UL) (or reverse) channels for transmitting information from the mobile unit to the base station. Multiple-input-multiple-output (MIMO) techniques may be employed when the base station and, optionally, the user terminals include multiple antennas. For example, a base station that includes multiple antennas can concurrently transmit multiple independent and distinct signals on the same frequency band to same user or multiple users in a cell/sector. MIMO techniques are capable of increasing the spectral efficiency of the wireless communication system roughly in proportion to the number of antennas available at the base station.
Conventional MIMO techniques coordinate operation of multiple antennas that are co-located with the coordinating base station. For example, the multiple antennas associated with a base station (BS) are typically configured so that the antennas are less than about 10 m from the base station. The signals transmitted from the base station to the antennas and then over the air interface to the mobile station (MS) on DL may be phase aligned so that they can be coherently combined at the receiver, e.g., the mobile station. Constructive and/or destructive interference of coherent radiation from the multiple antennas can therefore be used to amplify the signal in selected directions and/or null the signal in other directions. Processing of the coherent signals may also be used to minimize the mutual interference between multiple transmitters. Similarly on UL, signals received from multiple antennas can be combined to maximize signal strength, maximize SINR, detect multiple signals simultaneously through well-known algorithms such as MRC (maximum ratio combining), MMSE (minimum mean squared error), and MLSE (maximum likelihood sequence estimator). However, conventional MIMO does not address the inter-cell interference caused by uplink and/or downlink transmissions in neighboring cells.
A new class of multi-antenna techniques called Inter-Base Station MIMO (IBS-MIMO) has been proposed to enhance air-interface performance by enabling concurrent transmission of superposed signal waveforms from antennas at different base stations to one or more mobile terminals in such a way that the resulting mutual interference is suppressed. On the downlink, different BSs concurrently transmit (in a coordinated fashion) superposed signal waveforms from their antennas to one or more MSs in such a way that the resulting mutual interference is suppressed and the signals from multiple BSs may be coherently combined at each MS. In this process, the signal destined for a specific MS can be transmitted from different BSs. The radio access network provides control signaling and/or data plane exchanges to coordinate the BSs so that their transmissions can be coherently combined.
Coordination can be accomplished in different ways to create a range of IBS MIMO techniques. For example, coordination with the goal of achieving coherent reception at the MS for transmissions by a plurality of base stations while suppressing interference caused by transmissions to other MSs served by the same or different plurality of base stations is called “Network MIMO”. Network MIMO requires coordination across BSs at short time scales (e.g. on the order of a few ms to tens of ms). The actual time scales can be determined based on the maximum mobile terminal velocity expected to be supported. On the other hand, coordination to achieve non-coherent combining at the MS is called “Collaborative MIMO” and can be performed at longer time scales on the order of 100s of ms. Similar architectures can be used to support Network MIMO and Collaborative MIMO even though the two approaches impose different delay and bandwidth requirements on the network that connects the BSs.
Implementation of IBS-MIMO techniques is strongly constrained by existing network architectures and expected future developments in network architectures. Many network architectures support control plane operations and bearer plane operations. For example, base stations can be configured to handle bearer plane operations such as physical layer and medium access control layer operations. In some cases, the bearer (or data) plane can be separated into two levels: an upper level implemented in an IP gateway router and a lower level implemented in an IP-capable base station. Control plane operations such as scheduling and resource allocation may also be implemented in the base station. IBS-MIMO techniques should be implemented in a manner that is, to the greatest degree possible, consistent with these architectural constraints to minimize disruptions caused by implementation of these techniques.